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By attaching LEDs and lasers to moving objects, and shining their light on photochromic surfaces below them, we can display traces of many motions of interest in introductory physics courses and science museums.

We consider a demonstration in which pupils alternately slide and stop their fingers under a long horizontal rod which they support. The changeover is described in terms of the relevant kinetic and static friction. We present a model calculation, performed on a spreadsheet, which clarifies the process and describes graphically the stepwise switches as the fingers approach the centre of mass of the rod. (Contains 6 figures.)

We describe our experience in guiding a physics laboratory in the eleventh grade of a high school, in which regular laboratory classes are replaced by an experimental project carried out throughout the year. Some didactic suggestions and hints are given for those wishing to adopt such an undertaking. Outlines are given for a few of the recent projects. (Contains 5 figures.)

We physics teachers are forever in search of real-life applications of the theoretical concepts we teach. In mechanics we often utilize vehicle motion exercises, yet most textbook problems involving these are rather tame and deal with constant acceleration. What often captures the imagination of students is the actual performance of cars they drive: times for accelerating from zero to 100 km/h, top speed, energy consumption, power, and so on. Such data, of major concern to car designers and salesmen, are now readily available on the Internet and in magazines such as "Car & Driver" and "Road & Track" for a wide spectrum of vehicles. Their use in the classroom has been recommended in several articles that appeared in this journal. When reporting acceleration of vehicles, the first (high-acceleration) stage, as the engine "revs up" to its full power, is usually omitted. A glance at a typical velocity-versus-time curve (Fig. 1, for a MINI Cooper S; these data are also used in Figs. 3 and 5) shows that as the velocity increases toward the maximum attainable speed in a finite time, the acceleration decreases. Thus, overtaking a car at high speeds takes much longer than the initial acceleration might suggest--this is most unlike a constant acceleration scenario.

Optical elements manipulate light waves. They may be used to focus the light or to change the phase, the polarization, the direction, or the intensity of light. Many of these functions are often demonstrated with microwaves, since the devices normally available in teaching laboratories produce wavelengths in the centimeter range and are therefore more tangible. The same can be said for sound waves for frequencies in the kHz range. This property is the clue for producing elements such as half-wave plates, zone plates, and lenses for sound waves, based on techniques that have been developed for microwaves. This is the subject of the present article. Readers might wish to build some of these devices, which would then help to impress on students the unity of wave phenomena.

The physics of the plucked string has been treated in many articles and books. For our 12th-grade high school physics laboratory, we have built a cheap, simple sonometer apparatus for each pair of students on which they may investigate some interesting phenomena that arise when a string is plucked. Among these are the generation of harmonics (overtones) and the way their number depends on the length of a string and on where one plucks, the relation between the frequencies of the fundamental and those of the harmonics, and the way these are affected by changes in the length and the tension in the string. Such an experiment will help students appreciate the working of stringed musical instruments and, in particular, the contribution of overtones to the richness of sound produced.

Special events like science days, teacher's meetings and physics recruiting efforts require spectacular and, if possible, interactive experiments for the audience. Based on past experience with such events, we have gathered and present here a series of demonstration experiments in mechanics, optics, waves and electricity which are suitable, and supplement other efforts previously described in this journal. (Contains 20 figures.)

Incandescent lamps with tungsten filaments have been in use for about a century while being gradually replaced by fluorescent lamps; in another generation both will quite probably be largely replaced by light-emitting diodes. Incandescent lamps (simply called "lamps" in what follows) burn out after a lifetime that depends mostly on the temperature of the filament and hence the applied voltage. A full-term project (about 100 hours) on lamp burnout was carried out by two students in 1965 and has been briefly described. Many aspects of the physics of lamps have been dealt with in articles that have appeared in this journal, in the "American Journal of Physics," and in "Physics Education."

A take-home experiment in angular momentum used by the Massachusetts Institute of Technology is described. The experiment requires an inexpensive purchased motor kit, a hub and some washers.

A recent note in this journal presented a demonstration of standing waves along a cord consisting of two parts having different material densities, showing different sized wavelengths in each part. A generalization of that experiment to a continuously varying linear mass density is to vibrate a strip of material with gradually varying width (mass per unit length), in order to show that as the strip gets wider, the wavelength becomes shorter. Figure 1 shows the setup. It consists of a mechanical driver fed by a signal generator, and a strip of flexible fabric connected at its narrow end to the driver, nailed to a suitable piece of wood at the wider end. The strip is in the shape of a trapeze, 2 cm and 10 cm at its ends and some 50 cm long in its middle. It may be cut from the inner tube of a tire, or from body-building elastic strips.

Many physics teachers have a set of slides of single, double and multiple slits to show their students the phenomena of interference and diffraction. Thomas Young's historic experiments with double slits were indeed a milestone in proving the wave nature of light. But another experiment, namely the Poisson spot, was also important historically and in addition involves elements of drama and a clash between bold ideas of young scientists and the old guard--a recurrent theme throughout science. This article introduces wave optics with that experiment and relates the drama surrounding it in the year 1818. (Contains 3 figures.)

The photoelectric effect was discovered in the 19th century by Heinrich Hertz. It is interesting to note that the same scientist showed experimentally the wave nature of light, and thereby vindicated Maxwell's wave theory, and discovered the effect in which light shows corpuscular behavior. This paper describes a simple demonstration, enabling one to show the absence of a time lag in the photoelectric effect to an accuracy of a microsecond.

A mechanism for calculating the lifetime of an incandescent lamp as a function of applied voltage is proposed. A series of problems relating to the burnout of incandescent lamps is also presented.

An experiment uses horseshoe magnet and enamel wire among other things to observe the variation in alternating current by studying the oscillations caused by Lorentz Force.

The analysis of the diffraction of laser beams in different media, based on the density and the refractive index of the medium, is presented.

An electromagnetic experiment that was demonstrated at the 'cracking the safe' nationwide competition for Israeli high schools is presented.

Rotating shower of sparks can be created using simple birthday cake sparklers and other readily available material. Step-by-step instructions are provided on how to create rocket propulsion.

We have built a device that uses the Lorentz force on a current-carrying wire situated in a magnetic field, F = I L x B, in order to demonstrate a slowly varying alternating current by means of an optical lever. The apparatus consists of a horseshoe magnet, a length of thin enamel-coated wire (ours was 0.3 mm thick), a signal generator, a rheostat, and a laser. Construction is illustrated in the diagram (Fig. 1).

The bending of a laser beam in a medium with a density and refractive index gradient in the same direction has been described previously. When a transparent container is half filled with a salt or sugar solution and an equal amount of water is floated on top of it, then diffusion will create a concentration gradient from top to bottom. A laser light beam shone from the side of the container will then be bent gradually. Its path is shown in Fig. 1.

A study examining the invocation of concepts of diffraction and interference in the teaching of image formation, is presented.

The resonance in pipes, flasks and bottles is studied and explained with the help of Helmholtz resonators used for studying LRC circuits. The theoretical background and some experiments to describe the resonance factor are also discussed.

Science museums and popular physics shows often exhibit a blower in whose air stream a ball is held hovering in equilibrium some distance above the jet's orifice. The weight of the ball, "mg," is balanced by the drag force of the turbulent air stream, often written as ?Cv[superscript 2]A, where "?" and "v" are the density and velocity of the air, "A" is the effective area of the ball, and "C" some drag coefficient depending, among other things, on the surface roughness. This paper describes a ball and balloons of various sizes supported stably stacked one above the other in a vertical air stream from a blower.

Experiments to study the effect of Doppler beats, and subsonic and supersonic waves in ripple tank are explained.